This invention relates to a electron beam pattern transfer device having a photocathode mask and especially to the device and method for providing an accurate alignment between the mask and semiconductor wafer on which the mask pattern is transferred.
Due to the need for high integrated semiconductor circuits, various kinds of new lithographic techniques have been proposed, creating a breakthrough in conventional photo-lithographic techniques. One such technique employs electron beam transfer whereby an electron image projector, having a photoelectric surface mask, is used for transferring patterns from a mask to a semiconductor wafer. More specifically, the electron image projector reproduces high-resolution patterns onto a semiconductor wafer at 1:1 magnification. The desired pattern is imprinted on a quartz substrate mask, and a photocathode is then evaporated on the upper surface. The mask is then illuminated with an ultraviolet lamp which causes electrons to be emitted from the clear areas of the mask; thus the pattern is converted to electrons. The electrons are then accelerated by a high intensity electric field and focused onto the wafer by a magnetic field.
One of the major difficulties encountered in the development of an electron beam pattern transfer device has been the problem of aligning the mask and the wafer, which are usually spaced at approximately 10 mm. Conventional optical techniques such as contact aligners cannot be used since the mask and wafer must be separated to permit acceleration of the electrons. Utilizing secondary electrons which are employed for alignment purposes in conventional scanning beam systems cannot be incorporated in electron beam pattern transfer devices. Secondary electrons are usually trapped by the high intensity electric and magnetic fields and, thus, none will be detected by a detector. Another proposed method utilizes a very small hole formed in the wafer and detects the electrons passing through the hole. Forming such a hole in each wafer, however, is very troublesome and results in a loss of resist thickness and uniformity around the periphery of the hole; consequently, the integrity of the pattern transferred onto the wafer.
A still further proposed method utilizes a reference mark, consisting of heavy metal formed on the wafer, and detects the X-rays produced when photoelectrons are incident on this mark. Formation of this mark, however, is troublesome and the heavy metal, positioned on the wafer, has an adverse effect on the semiconductor's characteristics since it has a tendency of contaminating the device.